Chlorophylls in thin-film photovoltaic cells, a critical review

Chlorophyll dyes are widely spread in nature, and some of their semisynthetic analogs are available on the market. It is, therefore, very tempting to use these molecules in artificial photoconverting systems such as solar cells, thus avoiding problems with a labor-intensive synthesis of new photoactive components, their toxicity, biocompatibility and photostability. However, similar to a situation when mimicking a bird's flight does not necessarily lead to creation of a reliable aircraft, the attempts to make photovoltaically effective mimics of, for example, chlorophyll-based photosynthetic antenna have not so far ended up in commercial breakthroughs. This particularly concerns multilayer thin-film cells with planar or bulk molecular heterojunction(s), the type of devices that have been demonstrating a significant growth in power conversion efficiency over the past few decades. In this review we focus on those aspects of optical, self-assembling and bulk electrical properties of chlorophyll-type dyes that are of importance in solid-state photovoltaics. Starting from the very basics and tracing the developments in this field retrospectively, we come to a conclusion that the morphology of thin solution-processed films is a critical problem that arises immediately at the stage of manufacturing of a laboratory prototype. The measures that have been taken to date to improve the mobility of charge carriers both in the chlorophyll aggregates and throughout the photoconductive layer seem insufficient by comparison with their more successful counterparts like phthalocyanines.